This application relates to an improved valve-gated system for injection molding plastic, and more particularly to a new configuration of the hot runner passage leading to the gate.
It is well known that molecular orientation effects the strength of injection molded plastic products. Orientation is generally thought to be caused by the high shear stresses resulting from rapidly injecting the melt into the cavity and with a normal or conventional molding arrangement is usually parallel to the filling direction. This resulting unidirectional orientation causes the molded product to be stronger in the direction of orientation and weaker at right angles to the direction of orientation. For instance, a center-gated coffee cup would be stronger in the vertical direction, but very weak in the hoop direction. On the other hand, it is well known to form plastic film with a biaxial molecular orientation to improve its strength characteristics.
More recently, rotation of the mold has been used in injection molding to form the product with a biaxial or multiaxial molecular orientation. While this has been successful in achieving increased product strength, it is not a practical solution to the problem because of difficulties in constructing and operating the spinning molds. It is not suitable for multicavity applications, and is very difficult for irregular shaped products.
Even more recently, attempts have been made to achieve biaxial or multiaxial molecular orientation by designing the mold cavity to provide the inflowing melt with an irregular flow pattern. This has usually been in the form of ribs or other shapes against which the incoming melt impinges to spread or disperse it in different directions. While having better results in terms of increased product strength, this method or concept, of course, has the disadvantage that the product must include the ribs or other shapes introduced into the design to provide this filling action. Needless to say, this is not satisfactory for a large range of products.